This invention relates to a method and apparatus for preventing the inclusion of slag into the molten steel tapped from a converter.
In a steel refining by the converter, it is impossible to eliminate the occurrence of slag. When such slag is included in the molten steel in a tapping operation, such slag exists in the tapped molten steel as impurities and dephosphates the molten steel thus degenerating the quality of steel produced. The inclusion of slag also brings an ill-effect on the life of refractories of the vessel which receives the tapped molten steel.
For preventing the inclusion or mixing of slag into the molten steel in a tapping operation while assuring the sufficient yield of molten steel, it becomes necessary to stop the tapping of molten steel when the molten slag is about to be tapped from the tap hole.
As another problem, in the tapping operation, especially at the final stage of the tapping operation, the molten slag floating on the surface of the molten steel is whirled into the flow of the molten steel through the tap hole. It is also necessary to prevent the occurrence of such whirling phenomenon to increase the yield of the tapped molten steel.
Although the ratio of molten slag relative to the molten steel in the converter is different in each steel refining plant, the mean ratio is considered to be approximately 13.0 percent by weight (28.6 percent by volume). The inclusion amount of the slag into the tapped molten steel in the steel receiving ladle takes the values shown in graphs of FIG. 1 and FIG. 2. As readily understood from the graphs, the inclusion amount of slag spirally increases along with the widening of the tap hole.
FIG. 3 shows the relationship between the tapping time and the depth of the molten steel and molten slag above the tap hole in a converter which is gradually tilted to effect the tapping operation, wherein V.sub.1 indicates the amount of tapped molten steel free from molten slag and V.sub.2 indicates tapped molten slag received by the ladle.
The V.sub.1 and V.sub.2 amount can be calculated as follows: ##EQU1## From these calculation, EQU V.sub.1 :V.sub.2 =380:2.7.apprxeq.140:1
In the above formulae, A is the cross sectional area of tap hole, and C is the coefficient of fluidity.
From the above result of the calculation, it is assumed that, if the molten steel and molten slag are distinguished clearly from each other in the tapping operation, the amount of molten slag included in the tapped molten steel in the ladle can be 1/140 (by volume) of the tapped molten steel. However, as discussed above, the inclusion ratio of slag is about 1/20 in an actual tapping operation. This is reasoned as follows. Namely, in the final stage of the tapping operation, the molten steel is tapped from the tap hole in a situation as shown in FIG. 4, where a considerable amount of slag is whirled or included in the tapped molten steel, and such situation continues for several tens of seconds. However, the operator who observes such flow of tapped molten steel judges or considers that molten steel free from the slag is still being tapped from the converter.
Conventionally several methods and apparatuses have been proposed or developed for reducing the amount of slag inclusion in the tapped molten steel.
FIG. 5 shows one of such devices which the applicant of this invention has already disclosed in Japanese Patent Application No. SHO53-78910. The device is substantially characterized by disposing a stopper provided with a weir on the inner opening of the tap hole of a converter, subsequently detecting starting of the slag inclusion to the molten steel and finally dropping the stopper into the tap hole, whereby the discharge of slag is stopped. However, in this method, since the device is provided with the weir as well as the stopper, the structure thereof becomes extremely complicated, and furthermore, it is not possible to lift the stopper from the tap hole after dropping. Accordingly, there is a great possibility that a serious accident may occur by mismanipulation of the device. Still furthermore, since the device is mounted on a movable transport car, the operation before the converter is inconvenient.
FIG. 6 shows another method for preventing the inclusion of slag into the molten steel, wherein the method is characterized by the mounting of sliding nozzle device onto the tap hole of the converter.
This method has successfully reduced the slag inclusion to one fourth of the method which was available before the development of this method. However, in actual or practical operation, especially in terms of maintenance, the method has had several problems and it is less than optimal in preventing the inclusion of slag completely.
FIG. 7 shows still another method for preventing the inclusion of slag into the tapped molten steel, wherein the method is characterized by casting a refractory ball (usually referred to as "slag ball") onto the inner opening of the tap hole just before the starting of the inclusion of molten slag into the molten steel tapped from the tap hole. However, this method also has a problem since the self-setting of the slag ball toward the tap hole was expected in this method.
FIG. 8 shows still another conventional method for preventing the slag inclusion into the tapped molten steel. The method is characterized by mounting a field coil at the tap hole such that the tapped molten steel and slag pass through the coil and detecting the change of impedance between the molten steel and the molten slag and finally stopping tapping operation or splashing the molten slag from the tapped molten steel flow by air. This method can clearly distinguish the molten steel and the molten slag from each other by detecting the sharp difference of impedance, so long as the tapping is in stages where the molten steel and the molten slag flow out from the tap hole as a single-phase flow respectively. However, when the tapping operation is in a transitional stage where the molten slag is whirled into the molten steel flowing through the tap hole, the change of impedance is not apparent so that the detecting of either molten steel flow or molten slag becomes extremely difficult. Accordingly, in the above transitional stage which continues for several tens of seconds, although measures are taken to cope with the stage, for example, the air is applied to the slag-containing molten steel flow to remove the molten slag therefrom or the converter is tilted to stop the tapping of the molten steel, these measures cannot bring about the sufficient slag-cutting effect since no measures are taken for preventing the whirling of slag into the molten steel flow. Therefore, this measure is also less than optimal in complete slag-cutting and thereby suffering the poor yield of the tapped molten steel.
Several conventional methods have been discussed heretofore. However, these methods have disadvantages as well as advantages so that they are less than optimal in providing the satisfying slag-cutting result.
Accordingly, it is an object of the present invention to provide a method and apparatus which can overcome the above mentioned defects of the conventional methods and apparatuses, wherein the inclusion of molten slag into the molten steel tapped from the converter can be minimized, thus greatly enhancing the yield of the tapped molten steel.
The method and apparatus of this invention are, in summary, characterized in that the tap hole is closed by the stopper just prior to the starting of the whirling of the slag into the molten steel tapped through the tap hole and subsequently the tap hole is opened by lifting a stopper providing a steel-flow-out space between the stopper and the inner opening of the tap hole allowing the tapping of the molten steel through the space, whereby the mixing of the slag into the molten steel to be tapped can be reduced as small an amount as possible.
In this specification, the term "stopper" means any closure body (e.g. closure plug, closure bar) which has a contour suitable to close the inner opening of the tap hole completely.